Most of the ethylene produced in the world is made via the steam cracking process. This process usually consists of a feedstock (such as ethane, propane, butane, naphtha or gasoil) which is heated rapidly to high temperatures within tubular coils where the cracking reactions occur. The steam cracking furnace provides heat for the cracking reactions by burning fuel and transferring heat to the tubular coils which lie within the furnace firebox.
Steam is normally added to the feedstock in the coils prior to the radiant section of the furnace to provide the following benefits:
a) Reduce the hydrocarbon partial pressure within the coils to improve product yields. PA1 b) Reduce coking rate within the coils. PA1 c) Increase coil life by reducing carburization rate.
The steam cracking furnace is normally the key equipment item affecting profitability within a petrochemical plant. As such, much work has been done over the last 20 years to improve furnace performance; particularly feedstock flexibility, product yields and energy efficiency.
Product yields have been improved in recent years by reducing the residence time of the feedstock and products within the radiant section of the furnace and in the furnace coil outlet piping upstream of the quench points or Transfer Line Exchanger (T.L.E.)--see U.S. Pat. No. 3,923,921. At reduced residence times, coil average and coil outlet temperatures have increased to maintain feedstock conversion or cracking severity. At higher coil outlet temperatures, the need to very rapidly quench the cracking reactions becomes more important since this unfired residence time can result in rapid over-conversion of the feed and/or increased tar and coke formation. Current practice in the petrochemical industry is to locate quench points or T.L.E.'s relatively close to the furnace coil outlet and the hot furnace effluent is cooled/quenched to a point where most cracking reactions stop within a period of 30 to 50 milliseconds after exiting the furnace.
When the hot furnace effluent leaves the furnace, it can be quenched with an oil or water spray--see U.S. Pat. No. 4,599,478 and/or cooled using a T.L.E. Normal practice is that an oil spray is used when the cracking feedstock is gasoil or heavier and a T.L.E. is used for lighter feedstocks such as naphtha, L.P.G. and ethane. Using a T.L.E. is more energy efficient than oil quench since heat is recovered from the furnace effluent at a higher temperature level. Oil quench is normally employed for heavy feedstocks because the large tar and coke yields from them rapidly foul downstream equipment such as T.L.E.'s--see, for example, U.S. Pat. No. 4,444,697.
There are many T.L.E. designs and sometimes, in non-gasoil service, two T.L.E.'s are placed in series to extract the maximum amount of high level heat from the process stream. The first T.L.E. in a series is called the primary T.L.E. and the main functions of this exchanger are to very rapidly cool the furnace effluent and generate high pressure steam. The next T.L.E. is ca)led the secondary T.L.E. and its main functions are to cool the furnace effluent to as low a temperature as possible consistent with efficient primary fractionator or quench tower performance and generate medium to low pressure steam.
The drive towards higher energy efficiency within petrochemical plants in recent years has led to the development of T.L.E.'s that will cope with some gasoil feedstocks. These T.L.E.'s operate at higher temperatures than those in non-gasoil service and generate higher pressure steam to minimise the fouling caused by tar and coke deposition.
The deposition of coke within the cracking coil and in the quench points or T.L.E.'s is a major operating problem with steam cracking furnaces. The coke build-up finally limits furnace throughput (via a coil temperature constraint or unacceptably high pressure drops). The coke is removed by burning it off the metal surfaces (in an operation called decoking).
A major problem with existing cracking furnaces is the high coil outlet pressure that results from the pressure drop between the furnace coil outlet and the inlet of the process gas compressor; as the gas flows through piping, T.L.E.'s, fractionation and/or quench towers; and the safety requirement to maintain a process gas compressor suction pressure above atmospheric. Unfortunately this high pressure adversely affects the efficiency of the cracking reaction in the furnace. It has been recognised that a lowering of the pressure of the gas in the furnace outlet leads to improved product yields because there is a close correlation between the cracking reactions and the outlet gas pressure.
The present invention has as its principal object the provision of a motive fluid ejector, for lowering the furnace coil outlet pressure by compressing the furnace effluent to sufficiently high pressures at the ejector outlet to satisfy the pressure drop requirements of equipment between the ejector and the inlet to a process gas compressor, and at the same time to rapidly quench the temperature of the effluent gas on exiting the cracking furnace. A further objective is to control the quenching temperature so that the cracking reaction is stopped yet provides adequately high temperature effluent for efficient heat exchanger operation and less energy loss.
The present invention provides for relatively low furnace oil outlet pressures in the cracking furnace thus allowing relatively efficient cracking and therefore favourable product yields.
Accordingly, with the present invention, the amount of steam that is added to the coils prior to the radiant section of a steam cracking furnace may be significantly reduced with resultant energy savings.